The present application relates generally to root mean square (RMS) detectors and envelope detectors.
There are many applications in which it is desirable to measure the average power level of a radio frequency (RF) signal. For example, power measurement and control of RF signals in both the transmitting and receiving chains of modern wireless communications systems, such as cellular telephone networks, may be essential. To efficiently use the available bandwidth, the transmitted signals in these systems are modulated using complex modulation schemes such as Code Division Multiple Access (CDMA), Wideband Code Division Multiple Access (WCDMA), or Worldwide Interoperability for Microwave Access (WiMAX). These complex modulated signals have a time varying crest factor, which is defined as the peak to average power ratio of the signal. Intolerable errors can result if conventional power detectors using diode detection or successive amplification are used to measure the signal power.
Another challenge in modern wireless communication systems is improving the power efficiency of power amplifiers used in the transmit chain. Efficient use of power amplifiers is important in mobile communications systems. Improved power efficiency can provide significant benefits including lower overall operating costs. Improving the power efficiency of power amplifiers is especially difficult when high-crest factor signals (having a peak power of 10+dB more than the average power) are transmitted since the transmitter should be put in a deep back-off mode (very low average power output) to handle the linearity requirements for peak signal levels. To improve power efficiency, it is known to apply envelope tracking to the power amplifier input signal, and to use the detected envelope to vary the amplifier operation. For supplying power to the power amplifier, a variable power supply is utilized in an envelope tracking system. The input signal envelope power levels are monitored using an envelope detector, and the power that is supplied to the power amplifier is varied based on the monitored envelope levels. In particular, the supply voltage supplied to the power amplifier is varied so as to be just sufficient to reproduce the power level required by the amplifier at a given instant of time. Accordingly, at low envelope power levels, a low supply voltage is provided to the amplifier, and the full supply voltage is only provided when the maximum output envelope power is required, i.e., at the output power peaks.
RMS power detectors can precisely measure RF power independent of the modulation type (signal shape or crest-factor). Accurate RMS calculation of these complex modulation schemes requires long integration times to include the time-varying envelope in the measurement. Thus, commercially available RMS power detectors are generally not capable of providing the envelope level of the modulated signals.
Therefore, for transmitter systems, it is desirable to have a power detector that provides both average power information and input voltage envelope information.